The specific aims of the proposed study are: a. To evaluate the effectiveness of influenza vaccination in reducing influenza-associated morbidity and mortality among persons over age 65 during the periods of epidemic H3N2 influenza A in 1975-1976 and 1980-1981. b. To evaluate the effectiveness of multiple prior annual vaccination with a vaccine containing H3N2 antigen in reducing influenza-associated morbidity and mortality among persons over age 65 during the period of epidemic H3N2 influenza activity in 1975-1976 and 1980-1981. c. To perform these evaluations of influenza vaccination effectiveness for persons over age 65 with and without known underlying "high risk" medical conditions. The question of the effectiveness of influenza vaccine in reducing hospitalizations and deaths among the elderly is of critical importance to public health policy makers who formulate vaccination recommendations. The study will utilize a retrospective cohort design in which preepidemic influenza vaccination status of a defined population of elderly (Kaiser-Permanente Medical Care Program of Oregon) will be estimated; rates of treated morbidity and mortality occurring during specific epidemic periods among vaccinees and nonvaccinees will be ascertained from medical records; and these rates will be compared to evaluate effectiveness of vaccination. The proposed study involves the collaboration of two established investigators, a biostatistician and a medical epidemiologist, who have successfully completed vaccine effectiveness studies of earlier influenza epidemics using the KPMCP data system.